1. Field of the Invention
This invention relates to AGC systems and, in particular, to an AGC system for signal processing in concealed conductor locator devices.
2. Background
It is often necessary to locate buried conduits, which are employed by numerous utility companies, in order to repair or replace them. In addition, it is important to locate conduit lines in order not to disturb them when excavating for other purposes (such as, for example, addition of new conduits). Examples of buried conduits include pipelines for water, gas or sewage and cables for telephone, power or television. Many of the conduits are conductors, such as metallic pipelines or cables. In other applications, it is often useful to locate concealed elongated conductors, such as power lines or copper water lines, concealed in the walls of buildings. It is well known to locate concealed elongated conductors ("lines") by detecting electromagnetic emissions from them.
A conducting conduit (a line) may be induced to radiate electromagnetically by being directly or inductively coupled to an external transmitter. In some instances, such as with power lines, the line may radiate without an external transmitter.
A line locator detects the electromagnetic radiation emanating from the line. Early line locators included a sensor that detects a time-varying magnetic field. The line locator detects a peak amplitude signal or a minimum amplitude signal from the sensor, depending on the orientation of the sensor, when the line locator is passed over the line. Later line locators have included multiple sensors to provide further information to the line locator regarding the line (such as, for example, proximity information).
FIG. 1 shows a line 4, beneath surface 7, that is radiating a magnetic field 5. Magnetic field 5 is generally directed in a circular fashion around line 4. Line locator 1 is held over line 4 by operator 6. Line locator 1 includes sensor 3 that detects magnetic field 5 and displays a signal on a display 2 that is indicative of the magnetic signal strength measured by sensor 3. Depending on the orientation of sensor 3 (i.e., whether it is sensitive to horizontal or vertical components of magnetic fields), display 2 will indicate a maximum signal or a minimum signal when line locator 1 is held directly above line 4 (where the magnetic field 5 is directed horizontally).
FIG. 2A shows a line locator 200 having a top sensor 202 and a bottom sensor 201. Top sensor 202 and bottom sensor 201 are both horizontally oriented to detect magnetic fields oriented horizontally with respect to surface 7. Each of top sensor 202 and bottom sensor 201 has an output signal indicative of its position relative to the source of the time varying magnetic field 5, line 4.
FIG. 2B shows a block diagram of a circuit 217 for receiving and processing output signals from bottom sensor 201 and top sensor 202. The output signal from top sensor 202 is input to amplifier 203. The gain of amplifier 203 is set in response to a control signal G. The output signal from bottom sensor 204 is received by amplifier 204. The gain of amplifier 204 is set by control signal G. Switch 210 selects between the output signals from amplifier 203 or the output signal from amplifier 204. In operation, switch 210 is typically a momentary contact switch that normally selects the output signal from amplifier 204 (i.e., bottom sensor 201) and selects the output signal from amplifier 203 (i.e., top sensor 202) only when temporarily engaged. When switch 210 is engaged to receive output signals from top sensor 202, the control signal G is not allowed to vary. The output signal from switch 210 is filtered in filter 205. Filter 205 is typically a band-pass filter that passes signals of a particular frequency. In active operation, a current of a particular frequency is induced in line 4 by a transmitter (not shown) coupled to line 4. Typical frequencies of operation include 9.8 kHz and 82 kHz, but any convenient frequency can be used. In some instances, such as in a power line, line 4 may radiate electromagnetic radiation passively, i.e. without a transmitter.
The output signal from filter 205 is amplified in amplifier 206. The gain of amplifier 206 is set in response to control signal G. The output signal from amplifier 206 is converted to a DC signal in AC/DC converter 207 and amplified by DC amplifier 208.
The resulting output signal from circuit 217 is displayed to an operator (i.e., audibly with a speaker or visually through a meter) as an indication of the position of the line locator with respect to line 4. Engaging switch 210 so that the output signal from amplifier 203 is processed, in comparison with the processed output signal from bottom sensor 201, indicates the depth of line 4 below surface 7 or can be utilized for environmental noise compensation.
FIG. 2C shows the top signal and the bottom signal as a function of lateral distance from line 4 with control signal G held at a constant voltage. Often the signals from sensors 202 and 201 are weak and their absolute magnitude may vary over a wide range in amplitude (in some cases about 100 dB) in response to environmental parameters such as current strength, depth and soil type. In such a case, the control signal G, which is often manually adjusted by the operator, must continuously be adjusted to compensate for the widely varying magnitude of the signals.
In some cases, an automatic gain control (AGC) circuit 209 is used to produce a control signal G. In that case, an AGC circuit 209 is coupled to the output terminal of DC amplifier 208. In FIG. 2B, circuit 217 is shown to include AGC circuit 209, which controls the gain of amplifiers 203, 204 and 206 by varying control signal G so that the output signal from DC amp 208 is at a constant amplitude regardless of the output signal from sensor 201 or sensor 202. The useful information about magnetic field strength, then, is embedded in the control signal G.
However, the control signal G, determined by AGC circuit 209, is typically very nonlinear with the magnitude of the input signal, which often varies drastically with parameters. For example, the first 20 dB of input signal change takes two thirds of the available AGC signal and the next 80 dB of input signal change takes about one third of the available control signal range. The available control signal, outputted by AGC 209, is the total range of voltages that AGC 209 is capable of producing and to which amplifiers 203, 204 and 206 are capable of responding.
Processing useful information from the AGC determined control signal results in the use of highly complicated circuitry. Therefore, there exists a need for a line locator system with an AGC system where useful signals are more easily obtained.